IF you have raised the rev limiter, say to 7100 with a VAP tune, then the car will still shift at the old 6700 rev limit. As much of the extra power is high in the rev range, I prefer to shift manually.

 

400 extra RPMs above redline sounds a little dangerous. How much extra power is the motor putting out between 6700 and 7100? Do you need titanium valve springs to prevent floating at that RPM?

 

This is a 2014 V8 S composite chart over multiple days. The red line is the stock tune with a 6% upper pulley on 0W-20. The blue line is the same plus Racechip on 0W-40. In neither case does a late shift seem helpful.

My belief, which needs more research, is that the car was overheating and/or experiencing blow by with the pulley and pulling timing at high RPM. I went to 0W-40 for the next stage of boost which seems to have helped. The ambient temp was 96F on both days.
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The same dyno has several Z06s at ~545 WHP.